The invention relates to a method and apparatus for modulating a vestigial sideband (VSB) signal.
With the ever increasing proliferation of digital information, various standards have been established to outline the format for implementing the transmission of such digital information. For example, digital data may be transmitted as a sub-carrier in the lower vestigial sideband (VSB) spectral region of a standard television signal, e.g., a National Television Standards Committee (NTSC) broadcast video signal.
Specifically, xe2x80x9c8-VSBxe2x80x9d and xe2x80x9c16-VSBxe2x80x9d are two radio frequency (RF) modulation formats that are utilized by the digital television standard (e.g., Advanced Television Systems Committee (ATSC)) to transmit digital information terrestrially or via a cable environment to consumers respectively. An ATSC compliant transmitter will typically receive and modulate an encoded bitstream, e.g., an MPEG compliant bitstream, onto a RF channel and transmit the modulated signal to the consumers. With such wide acceptance of this digital technology, it is advantageous to obtain a transmitter having a low-cost digital VSB modulator that can be employed for high-definition television (HDTV) applications including remodulation of digital video directly to a typical television intermediate frequency (IF) or RF frequency.
Therefore, there is a need in the art for an apparatus and method for direct conversion of baseband digital symbols to a desired frequency (e.g., IF or RF) without the need for analog up conversion.
The invention is a method and apparatus for providing direct conversion of baseband digital symbols to a desired frequency (e.g., IF or RF) without the need for analog up conversion.
Specifically, the apparatus of the present invention is a VSB modulator comprising a complex frequency shifter, an interpolation filter, an optional pilot inserter, a DDS quadrature modulator and an optional image rejector and DAC compensator.
The input to the complex frequency shifter can be an ATSC bitstream having undergone FEC processing. At the input of the VSB modulator, the symbols are frequency shifted to generate a complex signal and then decimated to provide two half-rate streams.
The frequency-shifted signal is then received by the interpolation filter, where the filter is a polyphase 16:1 interpolation filter having real coefficients. Namely, interpolation filter is a baseband interpolation filter. The polyphase 16:1 interpolation filter effectively generates 16 output symbols for every input symbol.
The pilot inserter adds a complex pilot signal at xe2x88x922.69 MHz to the output of the interpolation filter. The pilot signal is a small DC shift that is applied as a helping signal. Namely, after frequency shaping the spectrum, the pilot signal is inserted at the left band edge. By performing pilot insertion after (rather than before) the interpolation filter, the datapath through the interpolation filter is maintained at 3 bits (rather than 16) so that the filter size is reduced. This is another advantage in reducing the overall cost of the modulator.
After pilot insertion, the data signal (a complex baseband signal) is applied to a quadrature modulator 240 that makes use of direct digital synthesizers (DDS) operating at 86.08 MHz. Two synthesizers are used to generate two real sinusoids that are 90 degrees out of phase from one another. Each of these sinusoids is multiplied by one of the input streams. Namely, the complex baseband signal to the quadrature modulator is digitally modulated up to some desired frequency, (e.g., intermediate frequency (IF) or RF). The desired frequency is selected within the DDS.
After multiplication, the two streams are multiplexed together to form a 176.16 MHz. output stream which contains the desired digital signal as well as images due to the synthesizer aliases. This stream is then applied to the image rejector and DAC compensator 250, which is tasked with performing image rejection and compensation for spectral rolloff in digital-to-analog converters.
Thus, the present invention provides an apparatus and method for providing direct conversion of baseband digital symbols to a desired frequency (e.g., IF or RF) without the need for analog up conversion. Additionally, the architecture of the present VSB modulator can be implemented using digital integrated circuit technologies, thereby significantly reducing the cost of devices that require a VSB modulator.